In order to face up to the increase in power which an alternator or alternator-starter of a motor vehicle must now provide because of the increased consumption by the on-board equipment, nominal on-board network voltages of approximately 42 V tend to be used instead of 14 V, in order to supply power levels of approximately 4 kW to 10 kW.
In addition, even if the nominal voltage of the on-board network remains 14 V, it may be desirable in some cases to increase the output performance of an alternator intermittently by increasing an intensity of an excitation current which circulates in a rotor of the alternator.
For this purpose, in patent application FR2802363, the company VALEO EQUIPEMENTS ELECTRIQUES MOTEUR proposes over-excitating the rotor by means of a voltage-increasing chopper circuit, from the on-board network.
A control loop makes it possible to maintain the excitation current at values which require the alternator to be permanently at a temperature lower than a maximum permissible temperature for the alternator and its components.
Another way of increasing the output performance of a standard alternator is to decrease the impedance of an excitation winding of the rotor, which has the effect of increasing the excitation current for the same nominal voltage of the on-board network.
This manner of proceeding is advantageous in a range-decreasing operation (known by the term downsizing), i.e. when it is wished to use in a vehicle of a certain category equipment of a vehicle of a lower category, by boosting its characteristics.
However, in the case of an alternator which is operating in over-speed mode, which is also known by the term boosting, the parts manufacturer clearly has the problem of thermal balancing of the machine, with the increase of the currents increasing correspondingly the losses such as the Joule losses and iron losses, for example.